Insights into Growth Behavior of Intermetallic Compounds in Sn-Ag-Cu Solder Joints during Mechanical and Thermo-Mechanical Deformation Processes

This work focuses on understanding the effect of strain rate imposed during various thermo-mechanical excursions (TMEs), such as thermal cycling and creep, and isothermal aging on the extent of coarsening of intermetallic compounds (IMCs) in diffusion bonded Sn-Ag-Cu(SAC)/Cu solder joints as well as reflowed bulk solder alloys. Fabricated samples were subjected to isothermal aging (IA), creep, thermal cycling (TC) and thermo-mechanical cycling (TMC) and the microstructure coarsening was monitored at regular intervals. A comparison has been made between thick joints of 1 mm thickness and micro-scale joints having a thickness of $\sim 150 \mu \mathrm{m}$. Both the interfacial IMC layer and the Ag3Sn precipitates in the bulk are observed to undergo significant coarsening during each of the TMEs. While the $150\ \mu \mathrm{m}$ joint showed a faster growth in the IMC layer thickness as compared to 1 mm joint in the initial stages, a drastic reduction in the layer thickness was also observed in both the joints in the latter stages. Such a behavior is believed to happen due to the breaking of the IMC layer into the solder, a phenomenon which can severely impact the reliability of the joint. It is also observed that cyclic shear strain rates have the maximum severity on the microstructure coarsening kinetics for all joint sizes. Finally, a comprehensive understanding of the effect of all the TMEs on the coarsening behavior and joint reliability is proposed.